1. Field of the Invention
The present invention relates to an electron gun, and in particular, to an electron gun characterized by a configuration of a field-emission type cold cathode and a cathode-ray tube using the same.
2. Description of the Related Art
FIGS. 9 to 11 show a conventional example of a cathode-ray tube apparatus equipped with a field-emission type cold cathode described in JP 9(1997)-82248 A. As shown in FIG. 9, an electron gun is mounted in a neck portion of the cathode-ray tube apparatus, and a field-emission type cold cathode 22 is provided in the electron gun. As shown in FIG. 10, the field-emission type cold cathode 22 is composed of a cold cathode 24 attached to a ceramic substrate 23, wiring from an emitter area 25 of the cold cathode 24, a bonding pad 26 for supplying an electric potential to the wiring, and a bonding wire 28 for electrically connecting the bonding pad 26 to an electrode 27. A focusing electrode 29 having an electron beam passage hole 30 is disposed downstream of the emitter area 25. As shown in FIG. 11, the emitter area 25 is composed of a silicon substrate 31, an insulating layer 32, a gate electrode 33, a plurality of hollows 34, and a plurality of emitters 35.
The thickness of the insulating layer 32 is about 1 xcexcm, the aperture diameter of the gate electrode 33 is about 1 xcexcm, and the tip end of each emitter 35 is pointed to be about 20 nm. The gate electrode 33 is supplied with a voltage of about 50 V with respect to the emitters 35. Because of this, a strong electric field (2 to 5xc3x97107 V/cm or more) is generated between the tip ends of the emitters 35 and the gate electrode 33, whereby electrons are emitted from the tip ends of the emitters 35. By arranging a number of minute cold cathodes with such a configuration in an array on the substrate 31, a flat cathode releasing a large current can be constituted.
The above-mentioned configuration is formed by a minute processing technique using a so-called semiconductor process. By mounting minute cold cathodes at high density, the cathode current density can be made 5 to 10 times that of a conventional hot cathode.
An attempt has been made to obtain a cathode-ray tube apparatus with high resolution by utilizing a high cathode current density to achieve a high density of electron beams and reduce the size of an electron beam spot formed on a phosphor screen of the cathode-ray tube apparatus.
In a cold cathode produced by such a minute processing technique, it is desired to decrease the production cost for a cold cathode by further reducing the size of the silicon substrate 31 of a cold cathode, thereby increasing the number of silicon chips to be produced from one wafer.
However, in the case where the size of the silicon substrate 31 is reduced, the emitter area 25 and the bonding pad 26 are disposed close to each other, and the bonding wire 28 to be arranged on the bonding pad 26 and the emitter area 25 also are disposed close to each other. Since the bonding wire 28 is disposed to be non-axially-symmetrical with respect to the emitter area 25, an electric field formed in the vicinity of the emitter area 25 is distorted to be non-axially-symmetrical. Therefore, electron beams passing through the electron beam passage hole 30 also are distorted to be non-axially-symmetrical, and the shape of an electron beam spot formed on a phosphor screen of the cathode-ray tube also is distorted. In the case where an electron beam spot is distorted, the resolution of the cathode-ray tube apparatus is degraded.
In order to solve the above-mentioned problem, JP 8(1996)-106848 A describes a cold cathode in which a cathode electrode having a plurality of electron-emission points, a gate electrode, and a focusing electrode are stacked while being insulated from each other, wherein a shielding electrode covering a feed terminal portion is opposed to and spaced from the focusing electrode. This configuration is different from the above-mentioned cold cathode configuration shown in FIGS. 10 and 11 to which the present invention is directed, in that the focusing electrode is stacked. However, in the configuration described in JP 8(1996)-106848 A, an attempt is made to prevent an electric field generated by a bonding terminal and wiring from influencing the track of electrons. Thus, the present invention and the subject matter of JP 8(1996)-106848 A share a common problem to be solved.
However, in the case where a shielding electrode is disposed in the above-mentioned cold cathode configuration to which the present invention is directed in a similar manner to that in JP 8(1996)-106848 A, since the bonding wire is arranged three-dimensionally, the distance between the cathode and the shielding electrode becomes large. Consequently, even if the shielding electrode is provided, the effect of minimizing the non-axial-symmetry of an electric field from the cathode to the aperture of the shielding electrode is small. Furthermore, when the bonding wire is disposed closer to the emitter area in connection with miniaturization of a cathode as described above, providing the shielding electrode has no effect on the disturbance of an electric field in the vicinity of the emitter area.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an electron gun and a cathode-ray tube capable of realizing high resolution without causing any distortion of an electron beam spot due to size reduction of a cathode at a low cost.
An electron gun of the present invention includes a cathode and a control electrode, wherein the cathode includes emitter tips provided on a substrate and supplied with a predetermined voltage, a gate electrode with an electric field formed between the gate electrode and the emitter tips, a terminal and a lead for supplying a voltage to the emitter tips, and a terminal and a lead for supplying a voltage to the gate electrode. A shield electrode further is provided between the cathode and the control electrode, and the shield electrode has a cylindrical projecting portion projecting toward the cathode, through which electron beams pass.
Herein, the term xe2x80x9ccylindrical projecting portionxe2x80x9d refers to a column body provided with a hollow portion through which an electron beam can pass, and its top and bottom surfaces do not necessarily have the same shape and size. Examples of the shape of the cylindrical projecting portion include a columnar shape in which generating lines are parallel to each other, a shape obtained by cutting the surface of a cone, in which generating lines pass through one point (apex), with two planes crossing all the generating lines and being parallel to each other, a bobbin shape in which a central portion is thin, etc.
According to the above configuration, the cylindrical projecting portion is provided so as to be integral with the shield electrode between the cathode and the shield electrode, whereby electron beams passing therebetween can be shielded by the projecting portion. Therefore, the electron beams are not influenced by an electric field generated by bonding wires. Furthermore, an electric field formed in the projecting portion is axially-symmetrical, so that the cross-section of an electron beam can be formed in a substantially perfect circle. Because of this, an electron beam spot on a phosphor screen is not distorted, and the cathode can be miniaturized.
Furthermore, it is preferable that an inner diameter of a tip end of the projecting portion is substantially equal to an inner diameter of a base thereof.
According to the above preferable configuration, the diameter of the electron beams further can be decreased in the vicinity of the cathode. Therefore, the distortion of an electron beam spot can be prevented, and the size thereof can be reduced.
Furthermore, it is preferable that an inner diameter of a tip end of the projecting portion is larger than an inner diameter of a base thereof.
According to the above preferable configuration, the diameter of electron beams further can be decreased in the vicinity of the cathode. Therefore, the distortion of an electron beam spot can be prevented, and the size thereof can be reduced.
Furthermore, a cathode-ray tube of the present invention includes a glass envelope composed of a front panel and a funnel, a phosphor screen formed on an inner surface of the front panel, and an electron gun disposed in a neck portion of the funnel, wherein the electron gun is the above-mentioned electron gun of the present invention.
According to this configuration, a cathode-ray tube capable of realizing high resolution without causing any distortion of an electron beam spot involved in size reduction of the cathode can be provided at a low cost.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.